When you cast a spell, you can consume all of your remaining spell slots to cast a spell at an extremely high level. Add up the levels of the spell slots consumed by this, and cast the spell at that level, even if you would not normally be able to cast a spell at that level. You can cast a spell at higher than 9th level this way.
After the spell ends, you take necrotic damage equal to the level of the spell. This damage ignores damage immunities and cannot be resisted in any way. In addition, if it was cast at a level higher than you could normally cast, you gain 3 levels of exhaustion. If the spell was cast at 9th level or higher, you gain 5 levels of exhaustion instead. After casting a spell in this way, you cannot regain any spell slots until you finish 2 Full days of rest.